From the past, carbon black and silica are widely used as reinforcing fillers for rubber (referred to hereinafter simply as reinforcing material). In general rubber is incorporated with such fillers widely by way of a dry process wherein a kneading apparatus such as a Banbury mixer, an open roll, or a kneader is employed.
According to the dry process, however, much more kneading energy and time are required to obtain a rubbery composition incorporated with fillers. Mentioned as the reason therefor is that a rubber such as a styrene-butadiene copolymer rubber or butadiene rubber is in the form of a bale so that tremendous shearing power is needed at the initial stage of kneading and a large amount of fillers is to be dispersed into such rubber that is difficult in kneading.
With a view to reducing kneading energy and time in the dry process, a process for obtaining a rubbery composition incorporated with fillers has been investigated wherein a rubber latex and fillers are mixed in an appropriate proportion, and then rubber in the rubber latex is coagulated with a coagulating agent such as an acid or a salt thereby incorporating the coagulated rubber uniformly with the fillers, or in other words, cocoagulation.
With respect to a process for obtaining a cocoagulation product of rubber containing silica as filler, there are proposed a method of treating silica with an alkyltrimethylammonium salt (patent literature 1), a method of dispersing silica together with a silylating agent into a rubber latex (patent literature 2), a method of treating silica with an organosilicon compound (patent literature 3) and a method of treating silica with a cationic polymer (patent literature 4).
In these patent literatures, however, there is no disclosure on a concrete manner from solid-liquid separation to drying of the resultant cocoagulation product and on the form of the resultant silica-filed rubber.
In case a method using a hot blast drier followed by a bale shaping, which is adopted as a general technique for conventional cocoagulation products of carbon black, is used for obtaining a dried product of the cocoagulation product of silica and rubber, the silica particles in the rubber are strong in their mutual action so that the dried product are obtained as an extremely hard block. A problem arises in such block since crushing followed by kneading of the block at the time of shaping is difficult. Looking at the production method, silica is of hydrophilic property and gives a high aqueous content in the resultant coagulation product so that a longer period of time is needed for drying, thus offering a problem in industrial operation.
These problems are caused by the fact that silica is extremely higher in hydrophilic property than carbon black.
On the other hand, there are disclosed a process for obtaining a powdery silica-filled rubber according to a method wherein an aqueous cocoagulation product obtained by cocoagulation of silica and rubber is subjected as such or after concentration to spray-drying, or alternatively, the aqueous cocoagulation product is well dried by the aid of a filter press or centrifugal separator and subjected to fluidized drying in a fluidized bed (patent literature 5) and according to a method wherein the cocoagulation product after solid-liquid separation is shaped into the silica-filled rubber in the form of pellets (patent literature 6).
However, the powder obtained according to the spray-drying has a relatively small average particle diameter of about 100 μm and easily becomes dusty. Further, the powder attaches to apparatus due to static electricity so that a problem arises in any weighing error.
The powders obtained by drying due to the fluidized bed are more or less adjustable in average particle diameter according to the degree of pulverization, but the particle size distribution of the resultant powders, including those attached each other in the fluidized bed and those more finely pulverized, becomes broader. Likewise the powder obtained by spray-draying, therefore, a problem also arises in the generation of dust caused by very fine power and in any weighing error caused by attaching of the power to apparatus due to static electricity.
The aforesaid pellets becomes appreciably hard after drying and their granular size is large, usually 5˜10 mm so that the effect of reducing shearing power at the initial stage of kneading is small. In case mutually attaching power of the pellets is strong, the pellets tend to initiate blocking during storage. Looking at the process for production, therefore, a problem arises in necessity of a separate step of making the pellets.
Patent literature 1: U.S. Pat. No. 4,482,657
Patent literature 2: JP-A-11-286577
Patent literature 3: JP-A-10-231381
Patent literature 4: JP-A-2003-113250
Patent literature 5: JP-A-2000-351847
Patent literature 6: JP-A-2003-160668